WASHINGTON — The US Supreme Court heard oral arguments Tuesday on the central issue in the challenge to President Barack Obama’s health care law: whether the US Constitution permits the federal government to require citizens to buy health insurance.

Justice Anthony Kennedy, a Reagan appointee seen by many as the potential swing vote in the case, appeared skeptical of the Obama administration’s arguments, saying the government has a “very heavy burden of justification” to show that the Constitution allows the federal government to coerce individuals to buy specific products.

Prominent Washington attorney and SCOTUSblog.com publisher Tom Goldstein said Tuesday that Kennedy’s position was “worrisome” for the Obama administration.

“It was hunting for a fifth vote and it was not obvious where that was going to come from,” Goldstein said, noting that the four Justices appointed by Democrats would likely vote in favor of the law.

But Goldstein told FOX News he still expected the Obama administration to ultimately prevail in the case.

Goldstein said that although the challengers appeared to have fared better in oral arguments, “in the end it won’t be enough, but it will be breathtakingly close.”

Tuesday’s hearing was the second of three days of arguments over Obama’s signature piece of legislation in what could be a landmark case on the scope of the federal government’s power to regulate interstate commerce.

The nine justices allotted two hours of oral arguments Tuesday on the constitutionality of the 2010 Patient Protection and Affordable Care Act’s “individual mandate” provision, which forces most Americans to purchase health insurance by 2014 or else pay a penalty.

The Obama administration has framed the individual mandate as part of a comprehensive reform law aimed at curbing surging health care costs and providing essential services to all Americans.

Solicitor General Donald Verrilli, who represented the administration before the court, argued that the individual mandate is valid under the Commerce Clause, the Necessary and Proper Clause, and the Taxing and Spending Clause of the US Constitution.

Verrilli said that the health care industry was different from other sectors in the economy because all individuals will require medical treatment at some point in their lives. He also told the court that the individual mandate was a valid regulation because the uninsured’s inability to pay for all of their health care bills has shifted costs to the insured by pushing up premiums.

But four of the Republican-appointed justices expressed concerns over what would prevent the federal government from mandating the purchase of other items if the individual mandate is upheld.

Chief Justice John Roberts said it may be impossible to set limits on Congress’ power if the court sides with the Obama administration. “All bets are off,” he said, according to The Wall Street Journal.

Justice Clarence Thomas, who has not asked a question during oral arguments in over six years, maintained his silence Tuesday but is seen by experts as a virtual lock to vote to strike the individual mandate down.

Lawyers for the 26 states and the National Federation of Independent Business (NFIB) challenging the PPACA followed Verrilli.

Clement was challenged on this point by Justice Ruth Bader Ginsburg, who compared the individual mandate to Social Security.

“There was a big fuss about that at the beginning,” she said, according to The Journal.

Justice Kennedy also made one of his few sympathetic remarks towards the Obama administration’s position when he took issue with Clement’s assertion that some individuals spend their entire lives without entering the health care or health insurance markets.

Kennedy pointed out that all citizens are in the health care market because they are creating a risk or injuring themselves or others by simply existing.

“I don’t think he was throwing anyone a bone,” Goldstein said of Kennedy’s question. “It is the one reason why he might vote for the government and he was articulating it.”

Michael Carvin, representing the NFIB, addressed the court after Clement and said that Verrilli’s argument that the health care industry was unlike any other was a “bogus limiting principle.” He said the court should instead uphold laws that regulate interstate commerce, but strike down laws that require citizens to enter into commerce.

“I can’t think of a brighter line than ours,” Carvin said, according to The Journal.

Obama signed the Patient Protection and Affordable Care Act (PPACA) into law just over two years ago without the support of a single congressional Republican. All four Republican presidential have vigorously opposed the law, with each sparring during debates over who would repeal the PPACA faster if elected president.

A decision by the court is due by late June, right in the heat of the 2012 election cycle.